mboned Working Group P. Savola
Internet Draft CSC/FUNET
Expiration Date: January 2005
B. Haberman
Caspian Networks
July 2004
Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address
draft-ietf-mboned-embeddedrp-07.txt
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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Abstract
This memo defines an address allocation policy in which the address
of the Rendezvous Point (RP) is encoded in an IPv6 multicast group
address. For Protocol Independent Multicast - Sparse Mode (PIM-SM),
this can be seen as a specification of a group-to-RP mapping
mechanism. This allows an easy deployment of scalable inter-domain
multicast, and simplifies the intra-domain multicast configuration as
well. This memo updates the addressing format presented in RFC 3306.
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 2004
problems but is not a complete solution for several reasons, as noted
below.
Further, it has been noted that there are some problems with the
support and deployment of mechanisms SSM would require [V6MISSUES]:
it seems unlikely that SSM could be usable as the only interdomain
multicast routing mechanism in the short term.
1.2. Solution
This memo describes a multicast address allocation policy in which
the address of the RP is encoded in the IPv6 multicast group address,
and specifies a PIM-SM group-to-RP mapping to use the encoding,
leveraging and extending unicast-prefix -based addressing [RFC3306].
This mechanism not only provides a simple solution for IPv6
interdomain Any Source Multicast but can be used as a simple solution
for IPv6 intradomain ASM with scoped multicast addresses as well. It
can also be used as an automatic RP discovery mechanism in those
deployment scenarios which would have previously used the Bootstrap
Router protocol (BSR) [BSR].
The solution consists of three elements:
o A specification of a subrange of [RFC3306] IPv6 multicast group
addresses defined by setting one previously unused bit of the
Flags field to "1",
o A specification of the mapping by which such a group address
encodes the RP address that is to be used with this group, and
o A description of operational procedures to operate ASM with PIM-
SM on these IPv6 multicast groups.
Addresses in the subrange will be called embedded-RP addresses.
This scheme obviates the need for MSDP, and the routers are not
required to include any multicast configuration, except when they act
as an RP.
This memo updates the addressing format presented in RFC 3306.
Some design tradeoffs are discussed in Appendix A.
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 20041.3. Assumptions and Scope
A 128-bit RP address can't be embedded into a 128-bit group address
with space left to carry the group identity itself. An appropriate
form of encoding is thus defined by requiring that the Interface-IDs
of RPs in the embedded-RP range can be assigned to be a specific
value.
If these assumptions can't be followed, either operational procedures
and configuration must be slightly changed or this mechanism can not
be used.
The assignment of multicast addresses is outside the scope of this
document; it is up to the RP and applications to ensure that group
addresses are unique using some unspecified method. However, the
mechanisms are very probably similar to ones used with [RFC3306].
Similarly, RP failure management methods, such as Anycast-RP, are out
of scope for this document. These do not work without additional
specification or deployment. This is covered briefly in Section 6.1.
1.4. Terminology
Embedded-RP behaves as if all the members of the group were all
intra-domain to the information distribution. However, as it gives a
solution for the global IPv6 multicast Internet, spanning multiple
administrative domains, we say it is a solution for inter-domain
multicast.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.5. Abbreviations
ASM Any Source Multicast
BSR Bootstrap Router
DR Designated Router
IGP Interior Gateway Protocol
MLD Multicast Listener Discovery
MSDP Multicast Source Discovery Protocol
PIM Protocol Independent Multicast
PIM-SM Protocol Independent Multicast - Sparse Mode
RIID RP Interface ID (as specified in this memo)
RP Rendezvous Point
RPF Reverse Path Forwarding
SPT Shortest Path Tree
SSM Source-specific Multicast
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 20042. Unicast-Prefix-based Address Format
As described in [RFC3306], the multicast address format is as
follows:
| 8 | 4 | 4 | 8 | 8 | 64 | 32 |
+--------+----+----+--------+----+----------------+----------+
|11111111|flgs|scop|reserved|plen| network prefix | group ID |
+--------+----+----+--------+----+----------------+----------+
Where flgs are "0011". (The first two bits have been yet undefined,
sent as zero and ignored on receipt.)
3. Modified Unicast-Prefix-based Address Format
This memo specifies a modification to the unicast-prefix-based
address format by specifying the second high-order bit ("R-bit") as
follows:
| 8 | 4 | 4 | 4 | 4 | 8 | 64 | 32 |
+--------+----+----+----+----+----+----------------+----------+
|11111111|flgs|scop|rsvd|RIID|plen| network prefix | group ID |
+--------+----+----+----+----+----+----------------+----------+
+-+-+-+-+
flgs is a set of 4 flags: |0|R|P|T|
+-+-+-+-+
R = 1 indicates a multicast address that embeds the address on the
RP. Then P MUST be set to 1, and consequently T MUST be set to 1, as
specified in [RFC3306]. In effect, this implies the prefix
FF70::/12.
The behavior is unspecified if P or T is not set to 1, as then the
prefix would not be FF70::/12. Likewise, encoding and the protocol
mode used when the two high-order bit in "flgs" are set to 11
("FFF0::/12") is intentionally unspecified until such time that the
highest-order bit is defined.
In the case that R = 1, the last 4 bits of the previously reserved
field are interpreted as embedding the RP interface ID, as specified
in this memo.
R = 0 indicates a multicast address that does not embed the address
of the RP and follows the semantics defined in [ADDRARCH] and
[RFC3306]. In this context, the value of "RIID" MUST be sent as zero
and MUST be ignored on receipt.
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 20044. Embedding the Address of the RP in the Multicast Address
The address of the RP can only be embedded in unicast-prefix -based
ASM addresses.
That is, to identify whether an address is a multicast address as
specified in this memo and to be processed any further, it must
satisfy all of the below:
o it MUST be a multicast address and have R, P, and T flag bits set
to 1 -- that is, be part of the prefix FF70::/12 (note that
FFF0::/12 is yet unspecified),
o "plen" MUST NOT be 0 (ie. not SSM), and
o "plen" MUST NOT be greater than 64.
The address of the RP can be obtained from a multicast address
satisfying the above criteria by taking the two steps:
1. copy the first "plen" bits of the "network prefix" to a zeroed
128-bit address structure, and
2. replace the last 4 bits with the contents of "RIID".
These two steps could be illustrated as follows:
| 20 bits | 4 | 8 | 64 | 32 |
+---------+----+----+----------------+----------+
|xtra bits|RIID|plen| network prefix | group ID |
+---------+----+----+----------------+----------+
|| \\ vvvvvvvvvvv
|| ``====> copy plen bits of "network prefix"
|| +------------+--------------------------+
|| | network pre| 0000000000000000000000 |
|| +------------+--------------------------+
\\
``=================> copy RIID to the last 4 bits
+------------+---------------------+----+
| network pre| 0000000000000000000 |RIID|
+------------+---------------------+----+
One should note that there are several operational scenarios (see
Example 3 below) when [RFC3306] statement "all non-significant bits
of the network prefix field SHOULD be zero" is ignored. This is to
allow multicast group address allocations to be consistent with
unicast prefixes, while the multicast addresses would still use the
RP associated with the network prefix.
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 2004
"plen" higher than 64 MUST NOT be used as that would overlap with the
high-order bits of multicast group-id.
When processing an encoding to get the RP address, the multicast
routers MUST perform at least the same address validity checks to the
calculated RP address as to one received via other means (like BSR
[BSR] or MSDP for IPv4). At least fe80::/10, ::/16, and ff00::/8
MUST be excluded. This is particularly important as the information
is obtained from an untrusted source, i.e., any Internet user's
input.
One should note that the 4 bits reserved for "RIID" set the upper
bound for RPs for the combination of scope, network prefix, and group
ID -- without varying any of these, you can 2^4-1 = 15 different RPs
(as RIID=0 is reserved, see section 6.3). However, each of these is
an IPv6 group address of its own (i.e., there can be only one RP per
multicast address).
5. Examples
Four examples of multicast address allocation and resulting group-to-
RP mappings are described here, to better illustrate the
possibilities provided by the encoding.
5.1. Example 1
The network administrator of 2001:DB8::/32 wants to set up an RP for
the network and all the customers, by placing it on an existing
subnet, e.g., 2001:DB8:BEEF:FEED::/64.
In that case, the group addresses would be something like
"FF7x:y40:2001:DB8:BEEF:FEED::/96", and then their RP address would
be "2001:DB8:BEEF:FEED::y". There are still 32 bits of multicast
group-id's to assign to customers and self ("y" could be anything
from 1 to F, as 0 must not be used).
5.2. Example 2
As in Example 1, the network administrator of 2001:DB8::/32 wants to
set up the RP, but to make it more flexible, wants to place it on a
specifically routed subnet, and wants to keep larger address space
for group allocations. That is, the administrator selects the least
specific part of the unicast prefix, with plen=32, and the group
addresses will be from the multicast prefix:
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 2004
FF7x:y20:2001:DB8::/64
Where "x" is the multicast scope, "y" the interface ID of the RP
address, and there are 64 bits for group-id's or assignments. In
this case, the address of the RP would be:
2001:DB8::y
The address 2001:DB8::y/128 is assigned to a router as a loopback
address and injected to the routing system; if the network
administrator sets up only one or a couple of RPs (and e.g., not one
RP per subnet), this approach may be preferable to the one described
in Example 1.
5.3. Example 3
As in Example 2, the network administrator can also assign multicast
prefixes like "FF7x:y20:2001:DB8:DEAD::/80" to some of customers. In
this case the RP address would still be "2001:DB8::y". (Note that
this is just a more specific subcase of Example 2, where the
administrator assigns a multicast prefix, not just invidial group-
id's.)
Note the second rule of deriving the RP address: the "plen" field in
the multicast address, 0x20 = 32, refers to the length of "network
prefix" field considered when obtaining the RP address. In this
case, only the first 32 bits of the network prefix field, "2001:DB8"
are preserved: the value of "plen" takes no stance on actual
unicast/multicast prefix lengths allocated or used in the networks,
here from 2001:DB8:DEAD::/48.
In short, this distinction allows more flexible RP address
configuration in the scenarios where it is desirable to have the
group addresses to be consistent with the unicast prefix allocations.
5.4. Example 4
In the network of Examples 1, 2 and 3, the network admin sets up
addresses for use by their customers, but an organization wants to
have their own PIM-SM domain. The organization can pick multicast
addresses like "FF7x:y30:2001:DB8:BEEF::/80", and then their RP
address would be "2001:DB8:BEEF::y".
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 20046. Operational Considerations
This section describes the major operational considerations for those
deploying this mechanism.
6.1. RP Redundancy
A technique called "Anycast RP" is used within a PIM-SM domain to
share an address and multicast state information between a set of
RP's mainly for redundancy purposes. Typically, MSDP has been used
for that as well [ANYCASTRP]. There are also other approaches, like
using PIM for sharing this information [ANYPIMRP].
The most feasible candidate for RP failover is using PIM for Anycast
RP or "anycasting" (i.e., the shared-unicast model [ANYCAST]) the RP
address in the Interior Gateway Protocol (IGP) without state sharing
(depending on the redundancy requirements, this may or may not be
enough, though). However, the redundancy mechanisms are outside of
the scope of this memo.
6.2. RP Deployment
As there is no need to share inter-domain state with MSDP, each
Designated Router connecting multicast sources could act as an RP
without scalability concerns about setting up and maintaining MSDP
sessions.
This might be particularly attractive when concerned about RP
redundancy. In the case where the DR close to a major source for a
group acts as the RP, a certain amount of fate-sharing properties can
be obtained without using any RP failover mechanisms: if the DR goes
down, the multicast transmission may not work anymore in any case.
Along the same lines, it's may also be desirable to distribute the RP
responsibilities to multiple RPs. As long as different RPs serve
different groups, this is trivial: each group could map to a
different RP (or sufficiently many different RPs that the load on one
RP is not a problem). However, load sharing one group faces the
similar challenges as Anycast-RP.
6.3. Guidelines for Assigning IPv6 Addresses to RPs
With this mechanism, the RP can be given basically any unicast
network prefix up to /64. The interface identifier will have to be
manually configured to match "RIID".
RIID = 0 must not be used as using it would cause ambiguity with the
Subnet-Router Anycast Address [ADDRARCH].
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 2004
If an administrator wishes to use an RP address that does not conform
to the addressing topology but is still from the network provider's
unicast prefix (e.g., an additional loopback address assigned on a
router, as described in example 2 in Section 5.1), that address can
be injected into the routing system via a host route.
6.4. Use as a Substitute for BSR
With embedded-RP, use of BSR or other RP configuration mechanisms
throughout the PIM domain is not necessary, as each group address
specifies the RP to be used.
6.5. Controlling the Use of RPs
Compared to the MSDP inter-domain ASM model, the control and
management of who can use an RP and how changes slightly and deserves
explicit discussion.
MSDP advertisement filtering typically includes at least two
capabilities: being able to filter who is able to create a global
session ("source filtering"), and being able to filter which groups
should be globally accessible ("group filtering"). These are done to
prevent local groups from being advertised to the outside, or
preventing unauthorized senders from creating global groups.
However, such controls do not yet block the outsiders from using such
groups, as they could join the groups even without Source Active
advertisement with a (Source, Group) or (S,G) Join by
guessing/learning the source and/or the group address. For proper
protection, one should set up, e.g., PIM multicast scoping borders at
the border routers. Therefore, embedded-RP has by default roughtly
equivalent level of "protection" as MSDP with SA filtering.
A new issue with control comes from the fact that nodes in a "foreign
domain" may register to an RP, or send PIM Join to an RP. (These have
been possible in the past as well, to a degree, but only through
willfull attempts or purposeful RP configuration at DRs.) The main
threat in this case is that an outsider illegitimately uses the RP to
host his/hers own group(s). This can be mitigated to an extent by
filtering which groups or group ranges are allowed at the RP; more
specific controls are beyond the scope of this memo. Note that this
does not seem to be a serious threat in the first place as anyone
with a /64 unicast prefix can create an own RP, without having to
illegitimately get it from someone else.
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 20047. The Embedded-RP Group-to-RP Mapping Mechanism
This section specifies the group-to-RP mapping mechanism for Embedded
RP.
7.1. PIM-SM Group-to-RP Mapping
The only PIM-SM modification required is implementing this mechanism
as one group-to-RP mapping method.
The implementation will have to recognize the address format and
derive and use the RP address using the rules in Section 4. This
information is used at least when performing Reverse Path Forwarding
(RPF) lookups, when processing Join/Prune messages, or performing
Register-encapsulation.
To avoid loops and inconsistancies, for addresses in the range
FF70::/12, the Embedded-RP mapping MUST be considered to be the
longest possible match and higher priority than any other mechanism.
It is worth noting that compared to the other group-to-RP mapping
mechanisms, which can be precomputed, the embedded-RP mapping must be
redone for every new IPv6 group address which would map to a
different RP. For efficiency, the results may be cached in an
implementation-specific manner, to avoid computation for every
embedded-RP packet.
This group-to-RP mapping mechanism must be supported by the RP, the
DR adjacent to the senders and any router on the path from any
receiver to the RP. Paths for Shortest Path Tree (SPT) formation and
Register-Stop do not require the support, as those are accomplished
with an (S,G) Join.
7.2. Overview of the Model
This section gives a high-level, non-normative overview of how
Embedded RP operates, as specified in the previous section.
The steps when a receiver wishes to join a group are:
1. A receiver finds out a group address from some means (e.g., SDR
or a web page).
2. The receiver issues an Mulicast Listener Discovery (MLD)
Report, joining the group.
3. The receiver's DR will initiate the PIM-SM Join process towards
the RP encoded in the multicast address, irrespective of
whether it is in the "local" or "remote" PIM domain.
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 2004
The steps when a sender wishes to send to a group are:
1. A sender finds out a group address using an unspecified method
(e.g, by contacting the administrator for group assignment or
using a multicast address assignment protocol).
2. The sender sends to the group.
3. The sender's DR will send the packets unicast-encapsulated in
PIM-SM Register-messages to the RP address encoded in the
multicast address (in the special case that DR is the RP, such
sending is only conceptual).
In fact, all the messages go as specified in [PIM-SM] -- embedded-RP
just acts as a group-to-RP mapping mechanism; instead of obtaining
the address of the RP from local configuration or configuration
protocols (e.g., BSR), it is derived transparently from the encoded
multicast address.
8. Scalability Analysis
Interdomain MSDP model for connecting PIM-SM domains is mostly
hierarchical in configuration and deployment, but flat with regard to
information distribution. The embedded-RP inter-domain model behaves
as if every group formed its own Internet-wide PIM domain, with the
group mapping to a single RP, wherever the receivers or senders are.
So, the inter-domain multicast becomes a flat, RP-centered topology.
The scaling issues are described below.
Previously foreign sources sent the unicast-encapsulated data to
their "local" RP, now they do so to the "foreign" RP responsible for
the specific group. This is especially important with large
multicast groups where there are a lot of heavy senders --
particularly if implementations do not handle unicast-decapsulation
well.
With IPv4 ASM multicast, there is roughly two kinds of Internet-wide
state: MSDP (propagated everywhere), and multicast routing state (on
the receiver or sender branches). The former is eliminated, but the
backbone routers might end up with (*, G) and (S, G, rpt) state
between receivers (and past receivers, for PIM Prunes) and the RP, in
addition to (S, G) states between the receivers and senders, if SPT
is used. However, the total amount of state is smaller.
The embedded-RP model is practically identical in both inter-domain
and intra-domain cases to the traditional PIM-SM in intra-domain. On
the other hand, PIM-SM has been deployed (in IPv4) in inter-domain
using MSDP; compared to that inter-domain model, this specification
simplifies the tree construction (i.e., multicast routing) by
removing the RP for senders and receivers in foreign domains, and
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 2004
eliminating the MSDP information distribution.
As the address of the RP is tied to the multicast address, the RP
failure management becomes more difficult, as the deployed failover
or redundancy mechanisms (e.g., BSR, Anycast-RP with MSDP) cannot be
used as-is. However, Anycast-RP using PIM provides equal redundancy;
this described briefly in Section 6.1.
The PIM-SM specification states, "Any RP address configured or
learned MUST be a domain-wide reachable address". What "reachable"
precisely means is not clear, even without embedded-RP. This
statement cannot be proven especially with the foreign RPs as one can
not even guarantee that the RP exists. Instead of manually
configuring RPs and DRs (configuring a non-existent RP was possible
though rare), with this specification the hosts and users using
multicast indirectly specify the RP themselves, lowering the
expectancy of the RP reachability. This is a relatively significant
problem but not much different from the current multicast deployment:
e.g., MLDv2 (S,G) joins, whether ASM or SSM, yield the same result
[PIMSEC].
Being able to join/send to remote RPs raises security concerns that
are considered separately, but it has an advantage too: every group
has a "responsible RP" which is able to control (to some extent) who
are able to send to the group.
A more extensive description and comparison of the inter-domain
multicast routing models (traditional ASM with MSDP, embedded-RP,
SSM) and their security properties has been described in [PIMSEC].
9. Acknowledgements
Jerome Durand commented on an early draft of this memo. Marshall
Eubanks noted an issue regarding short plen values. Tom Pusateri
noted problems with an earlier SPT-join approach. Rami Lehtonen
pointed out issues with the scope of SA-state and provided extensive
commentary. Nidhi Bhaskar gave the draft a thorough review.
Toerless Eckert, Hugh Holbrook, and Dave Meyer provided very
extensive feedback. In particular, Pavlin Radoslavov, Dino
Farinacci, Nidhi Bhaskar, and Jerome Durand provided good comments
during and after WG last call. Mark Allman, Bill Fenner, Thomas
Narten, and Alex Zinin provided substantive comments during the IESG
evaluation. The whole MboneD working group is also acknowledged for
the continued support and comments.
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 200410. Security Considerations
The addresses of RPs are encoded in the multicast addresses -- and
thus become more visible as single points of failure. Even though
this does not significantly affect the multicast routing security, it
may expose the RP to other kinds of attacks. The operators are
encouraged to pay special attention to securing these routers. See
Section 6.1 on considerations regarding failover and Section 6.2 on
placement of RPs leading to a degree of fate-sharing properties.
As any RP will have to accept PIM-SM Join/Prune/Register messages
from any DR, this might cause a potential Denial of Service attack
scenario. However, this can be mitigated by the fact that the RP can
discard all such messages for all multicast addresses that do not
encode the address of the RP. Both the sender- and receiver-based
attacks are described at more length in [PIMSEC].
Additionally the implementation SHOULD also allow manual
configuration of which multicast prefixes are allowed to be used.
This can be used to limit the use of the RP to designated groups
only. In some cases, it is desirable to be able to restrict (at the
RP) which unicast addresses are allowed to send or join to a group.
(However, note that Join/Prune messages would still leave state in
the network, and Register messages can be spoofed [PIMSEC].)
Obviously, these controls are only possible at the RP, not at the
intermediate routers or the DR.
It is RECOMMENDED that routers supporting this specification do not
act as RPs unless explicitly configured to do so; as becoming an RP
does not require any advertisement (e.g., through BSR or manually),
otherwise any router could potentially become an RP (and be abused as
such). Further, multicast groups or group ranges to-be-served MAY
need to be explicitly configured at the RPs, to protect from being
used unwillingly. Note that the more specific controls (e.g.,
"insider-must-create" or "invite-outsiders" models) to who is allowed
to use the groups are beyond the scope of this memo.
Excluding internal-only groups from MSDP advertisements does not
protect the groups from outsiders, only offers security by obscurity;
embedded-RP offers similar level of protection. When real protection
is desired, e.g., PIM scoping should be set up at the borders; this
is described at more length in Section 6.5.
One should observe that the embedded-RP threat model is actually
rather similar to SSM; both mechanisms significantly reduce the
threats at the sender side. On the receiver side, the threats are
somewhat comparable, as an attacker could do an MLDv2 (S,G) join
towards a non-existent source, which the local RP could not block
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 2004
Values 64 < "plen" < 96 would overlap with upper bits of the
multicast group-id; due to this restriction, "plen" must not exceed
64 bits. This is in line with RFC 3306.
The embedded-RP addressing could be used to convey other information
(other than RP address) as well, for example, what should be the RPT
threshold for PIM-SM. These could be, whether feasible or not,
encoded in the RP address somehow, or in the multicast group address.
In any case, such modifications are beyond the scope of this memo.
For the cases where the RPs do not exist or are unreachable, or too
much state is being generated to reach in a resource exhaustion
Denial of Service attack, some forms of rate-limiting or other
mechanisms could be deployed to mitigate the threats while trying not
to disturb the legitimate usage. However, as the threats are
generic, they are considered out of scope and discussed separately in
[PIMSEC].
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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Internet Draft draft-ietf-mboned-embeddedrp-07.txt July 2004
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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